drm/i915/gvt: Add mmio iterator intel_gvt_for_each_tracked_mmio()

This patch add a function intel_gvt_for_each_tracked_mmio() to
iterate each tracked mmio. The caller don't be aware of how the
tracked mmios are presented internally.

v2: remove snapshot_hw_mmio_registers().

Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>